1. Field of the Invention
The present invention relates to an apparatus and method for forming a latent image on a photo receptor belt.
2. Description of the Related Art
There have been developed devices for forming a color image using a photo receptor belt on which a latent image will be formed for each color. In such a color image forming device, there is a possibility of the photo receptor belt moving like a snake, resulting in displacements of the photo receptor belt in the primary scanning direction. Since such movements of the photo receptor belt cause displacements of a latent image for each color, it is very important to precisely keep the writing start positions for colors at the same position in the primary scanning direction on the photo receptor belt. To prevent color deviation, several image forming apparatuses have been proposed as described hereafter.
A conventional image forming apparatus is disclosed in Japanese Patent Unexamined Publication No. 4-181276. A photo-sensor is used to detect displacements of the photo receptor belt in the primary scanning direction. The photo-sensor is composed of a number of sensor elements arranged in a line and is placed in the inner side at an edge portion of the photo receptor belt such that the end portion of the photo-sensor protrudes from the edge line of the photo receptor belt. Therefore, the photo-sensor can detect the laser beam traversing only a range from the end thereof to the edge of the photo-receptor belt. The distance from the end thereof to the edge of the photo receptor belt is measured by counting the number of sensor elements therebetween. On a first turn of the photo receptor belt, a measured count value is stored. When the photo receptor belt is turned for a second time, a count difference between the first measured count value and the second measured count value is calculated. The count difference is used to determine the write start position on the photo receptor belt.
Another conventional image forming apparatus is disclosed in Japanese Patent Unexamined Publication No. 5-119574. A plurality of primary scanning laser units are provided to form latent images on the photo receptor belt, respectively. A photo-sensor for each primary scanning laser unit is placed to detect a pre-pattern formed on the edge portion of the photo receptor belt when the photo receptor belt is scanned with a laser beam by the primary scanning laser unit. The position detection signal of the photo-sensor is used to determine the write start timing.
However, in the case of the conventional apparatus using the count difference, a memory for storing the count difference is needed and further a controller is burdened with the distance measurement and displacement calculation.
On the other hand, in the case of the conventional apparatus using pre-pattern position detection signals, it is necessary to provide the photo receptor belt with the pre-patterns. Further, since the write start timing is determined using only the pre-pattern detection signal, the write start timing cannot be determined with sufficient precision.